1. Field of the Invention
This invention relates generally to an apparatus and method for testing the surface mounted modules containing integrated circuit (IC) chips. More particularly, this invention relates to a testing fixture for convenient mounting the packaged monolithic microwave integrated circuits (MMICs) thus facilitating precise testing of the packaged MMICs to improve the quality control and assurance and to increase the production through-put thus achieving higher level of productivity.
2. Description of the Prior Art
Because of recent progress made in the semiconductor technology and packaging engineering, all the circuit elements of a high frequency RF signal processor can now be incorporated in a miniaturized MMIC package. Furthermore, the demand for large quantity of MMIC packages is also increased rapidly as these miniaturized packages are being used for broader range of applications in the field of communication, signal processing and other microwave transmission applications. However, mass production of high quality monolithic microwave integrated circuits (MMICs) is still limited by the difficulty that the speed of testing these MMIC packages is very slow because such tests often involve time consuming manual processes.
Conventional testing processes involve soldering the microwave package to a testing printed circuit (PC) board whereby the ports on the package are securely connected to the appropriate testing circuits which are then mechanically locked to a set of signal measuring assemblies (SMAs) for further connecting to co-axial cables for the performance of a sequence of tests. After the tests, a set of reverse operations must be performed to disassemble the MMIC package from the SMAs and then from the testing PC board. The processes of soldering the package to-the PC board and locking the soldered combination of the PC board and the MMIC package to the SMAs and then the disassembling processes are very time consuming. Furthermore, the MMIC packages are often damaged during the tests while the testing PC boards, and the SMAs are also limited by very short life cycles because of the heavy duty imposed by such assembling and disassembling procedures which must be repeated performed.
The conventional testing procedures are also limited by the fact that the accuracy of the test results is limited by the losses caused by the transition and the impedance mismatch between the MMIC package and the testing PC board. The accuracy of the testing is degraded when the impedance deviates from the matching impedance which is generally an impedance of fifty-ohms (50-.OMEGA.). The transition losses also prevent the measurement results to reflect the true performance of the MMIC package.
There are on-going efforts to improve the testing fixtures and the testing processes in order to improve the testing accuracy and the speed of the tests. Hirsch et al. disclose in U.S. Pat. No. 4,897,601 entitled "Test Fixture for Integrated Circuit Chips" (Issued on Jan. 30, 1990) an MMIC test fixture wherein an MMIC chip is placed between two chip stops and onto a planar surface. These two chip stops each having a wedging surface which asserts a predetermined clamping force onto the edges of the MMIC chip thus pressing the chip into grounding contact with the testing fixture. The MMIC chip is then heated to bond with the RF and bias connection on the testing fixture while the tests are performed. After the tests are completed, the MMIC chip is removed from the bonded connections by a heating process and the clamping force is release by a leverage means on one the chip stops to remove the MMIC chip from the testing fixture.
A nondestructive testing process is disclosed by Hirsch et al. which may reduce the damage of the MMIC chip caused by the testing process by providing a predetermined clamping force. However, the operation of the chip stops still involves bolting of many threaded screws on the chip stops to the base of the test fixture. The speed of the tests are not improved due to the fact that such bolting operations are time consuming process.
Additionally, the loss incurred by the use of this testing fixture is worsened because instead of the micro-strips provided in the testing PC board, the testing fixture as disclosed has outwardly extended micro-strips across the elongated the test fixture. The additional lengths of these micro-strips may cause further deviations of the testing results in transmitting the test signals through these strips to the MMIC circuit.
Romanofsky et al. disclose in U.S. Pat. No. 4,980,636 entitled "Universal Nondestructive MM-Wave Integrated Circuit Test Fixture" (Issued on Dec. 25, 1990) an MMIC test fixture wherein bias module with spring-loaded contacts are used to electrically engage the pads on an MMIC chip carrier. The MMIC chip carrier is disposed in a recess of a base member of the testing fixture. RF energy is applied to and passed from the chip carrier by chamfered edges of ridges in the wave guide passages of two removable housings. These two removable housings are then attached to the base member containing the MMIC chip carrier by a plurality of bolts. Different types of calibration standard cards including the `Thru`, `Delay`, and `Short` are then inserted individually to perform different functional tests.
The test fixture as disclosed by Romanofsky et al. again suffers the same limitation that time consuming process of bolting the testing modules to the base member is required. These bolting procedures must be repeated several times because there are several inserts which must be changed when a set of test with one insert is completed. The time required for testing the MMIC chips would therefore limits the productivity of such chips.
Furthermore, the testing fixture disclosed by Romanofsky et al. utilizes removable housings which have wave guide passage including chamfered edges of ridges for applying RF energy to perform the test. These ridges are uniquely designed and fabricated for testing of special chips. The shape, length, and material for making these curved ridges are not fully described and since the curvature of these ridges are frequency dependent as is indicated in the Patent, the testing fixture disclosed by Romanofsky would probably be limited in its use for specific type of MMIC chip. Since the characteristics of these ridges have to be redesigned and re-fabricated, the usefulness of the testing fixture is limited by the efforts required to make the right: kinds of testing fixture appropriate for specific type of MMIC chips. These efforts would probably involve many trial and error with ridges of different shape and curvatures before a right kind is determined. Such uncertainty and risk in design and fabricating the testing fixtures would certainly limit the application of the apparatus disclosed by Romanofsky et al.
In order to overcome the aforementioned difficulties encountered by the Romanofsky et al.'s Patent, Tektronix use a testing PC board, i.e., Tektronix's RT/Duroid board, which has many micro-strips on its testing surface. These micro-strips are formed by the thin film and etching technology which can very precisely fabricated to match the impedance of the MMIC chip. These micro-strip are connected between the test ports on the MMIC boards and the external co-axial cables to perform the test. Tektronix's technique, i.e., the TKIT14E evaluation kit, however is limited by the excessive losses including the transition loss and the loss across the micro-strips on the PC board. The results of the MMIC tests obtained by Tektronix's evaluation kit are indirect and often cannot be reproducible due to the fact that the input impedance cannot be accurately controlled. Furthermore, the usefulness of this evaluation kit is limited to testing of the MMIC package that have extended leads which can be connected to the micro-strips.
Therefore, there is still a need in the art of MMIC test to provide an apparatus and method such that the operational characteristics of a an MMIC chip in different frequency range can be precisely measured in a well controlled and reproducible manner to assure high quality of MMIC chips can be consistently produced. For the purpose of enabling mass production of these high quality MMIC chips, the apparatus and method must be reliable and simple to use such that the testing process would not become too complicate and that the time required for performing the tests can be reduced to a reasonable level whereby it would not become a bottleneck in the fabrication process.